This book shows that inequalities in children's cognitive abilities are substantial from the beginning, with disadvantaged children starting kindergarten with significantly lower cognitive skills than their more advantaged counterparts. These same disadvantaged children are then placed in low-resource schools, magnifying the initial inequality. These conclusions are based on analysis of the U.S. Department of Education's Early Childhood Longitudinal Study, Kindergarten Cohort, a recent and comprehensive data collection effort that provides a nationally representative picture of kindergarten students. The book reports differences in young children's achievement scores in literacy and mathematics by race, ethnicity, and socioeconomic status (SES) as they enter kindergarten. It also explores differences by social background in a wide array of children's family and home conditions and activities. The analysis leads to several conclusions relevant to education policy: there are substantial differences by race and ethnicity in children's test scores as they begin kindergarten; race and ethnicity are associated with SES; family structure and educational expectations have important associations with SES, race/ethnicity, and test scores; SES strongly relates to cognitive skills; and low-SES children begin school in systematically lower-quality elementary schools than do their more advantaged counterparts. (Contains 31 bibliographic references.) (SM)